Boost thread priority when loading a DLL on a background thread

As described in the linked bug, loading DLL on a background thread may
cause a priority inversion on the loader lock. It is a common source of
jank in Chrome.

Bug: 973868
Change-Id: Ic375b0b4e94cca763f7c48ebb3c231ac5a1223a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2071960Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744526}

device/bluetooth/bluetooth_adapter_winrt.cc

@@ -30,6 +30,7 @@
 #include "base/strings/utf_string_conversions.h"
 #include "base/task/post_task.h"
 #include "base/task/task_traits.h"
+#include "base/threading/scoped_thread_priority.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/win/core_winrt_util.h"
 #include "base/win/post_async_results.h"
@@ -706,6 +707,10 @@ void BluetoothAdapterWinrt::InitForTests(
 // static
 BluetoothAdapterWinrt::StaticsInterfaces
 BluetoothAdapterWinrt::PerformSlowInitTasks() {
+  // Mitigate the issues caused by loading DLLs on a background thread
+  // (http://crbug/973868).
+  SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY();
+
   if (!ResolveCoreWinRT())
     return BluetoothAdapterWinrt::StaticsInterfaces();